add debugging method for MemMapFs

memmap.go

diff --git a/memmap.go b/memmap.go
index 88c390f..327aea1 100644
--- a/memmap.go
+++ b/memmap.go
@@ -369,3 +369,9 @@
 		fmt.Println(x.Name(), y.Size())
 	}
 }
+
+func debugMemMapList(fs Fs) {
+	if x, ok := fs.(*MemMapFs); ok {
+		x.List()
+	}
+}